Coaxial cables are widely used to carry high frequency electrical signals. Coaxial cables have a relatively high bandwidth and low signal losses, are mechanically robust, and are relatively low cost. A coaxial cable typically includes an elongate inner conductor, a tubular outer conductor, and a dielectric separating the inner and outer conductors. The dielectric may be, for example, a plastic foam material. An outer insulating jacket may be applied to surround the outer conductor.
One particularly advantageous use of coaxial cable is for connecting electronics at a cellular or wireless base station to an antenna mounted at the top of a nearby antenna tower. For example, the transmitter and receiver located in an equipment shelter may be coupled via coaxial cables to antennas carried by the antenna tower. A typical installation includes a relatively large diameter main coaxial cable extending between the equipment shelter and the top of the antenna tower to thereby reduce signal losses. For example, CommScope, Inc. of Hickory, N.C., offers its CellReach® coaxial cable for such applications.
With respect to such large diameter main coaxial cables in particular, CommScope typically uses a composite inner conductor that includes a dielectric rod surrounded by a conductive tube. Since the skin depth at the operating frequencies is relatively shallow, the conductive tube can be used to reduce costs and provide good mechanical properties. The conductive tube is typically formed by shaping a metal strip into a tube and welding the longitudinal seam. The dielectric rod also acts to block moisture within the tube.
U.S. Pat. No. 6,326,551 to Adams discloses a coaxial cable having a composite core comprising a welded tubular inner conductor with a water absorbing material therein. The composite core not only supports the cable during bending and promotes the maintenance of good signal transmission performance, but also protects against corrosion causing moisture getting into the cable.
The manufacture of such a coaxial cable thus usually entails not only a separate step of pre-forming the dielectric rod, but also properly positioning it relative to a conductive strip or other material from which the conductive tube is to be formed. Such multi-step manufacturing can be complex and time consuming. Accordingly, it can also add considerably to the costs of manufacturing a coaxial cable with a composite core.